This invention relates to speech recognition, and in particular to a system that can detect that a pass phrase has been spoken, and may also be able to validate that the pass phrase was spoken by a specified speaker, allowing the system to be used as a hands-free and low power consumption means of activating higher power consumption functions such as speech recognition in consumer devices, with smartphones being just one example of such consumer devices. Aspects of the invention also relate to an analog-to-digital converter suitable for use in such a system or device.
It is known to provide circuitry which is able to continually listen for voice commands, while in stand-by mode. This removes the requirement for a button or other mechanical trigger to generally ‘wake up’ the device from stand-by mode, for instance to activate a speech recognition function.
One possible way of initiating hands-free operation is for the user of the phone to say a key phrase, for example “Hello phone”. The device is then able to recognise that the key phrase has been spoken and wake up the speech recognition function and potentially the rest of the device. Furthermore the hands-free command may be programmed to be user specific, in that case only a previously registered user (or users) can utter the key phrase and the device will be able to verify that it is that specific user speaking (speaker recognition) and progress to wake up the speech recognition function.
However, such circuitry implements relatively complex algorithms and thus has relatively high power consumption, meaning that it is somewhat impractical to keep such circuitry continually active in a portable device with limited battery capacity or other power availability constraints.
In order to perform digital processing of sounds, it is usually necessary to detect the sounds with a microphone, which generates an analog signal, and to perform analog-to-digital conversion to generate a digital signal in a form that is suitable for such processing.
To provide sufficient accuracy in the digitisation of the speech signal for reliable speech recognition or user recognition, a high performance analog-to-digital converter (ADC) is required. Typically this will include some single-bit or multi-bit quantiser embedded in a feedback loop to spectrally shape the quantisation noise, for example as a delta-sigma ADC.
The quantiser may take several forms, including a voltage-to-frequency converter (or voltage-controlled-oscillator (VCO)) followed by a counter. The VCO generates a pulse train at a frequency that depends on the present value of its input analog voltage signal. The counter may count the number of pulses generated by the voltage controlled oscillator in a given time interval. The digital count value accumulated during each interval thus depends on the frequency at which the pulses are generated, and is thus a measure of the present value of the analog signal presented to the quantiser.
The ADC feedback loop will typically also comprise a digital-to-analog converter (DAC) to provide an analog feedback signal from the quantiser digital output and an analog op amp integrator to accept this feedback signal and the input signal. These analog components must have low thermal noise and adequate speed, and as a result they will consume relatively high power. Again, it is somewhat impractical to keep such ADC circuitry continually active in a portable device with limited battery capacity or other power availability constraints.